Research on center-assisted ring-core few-mode fiber with an eccentric circle for mode degeneracy separation in space division multiplexing

An eccentric-circle-assisted ring-core fiber (ECRF) structure is proposed to effectively separate spatial-degenerated linear polarization (LP) modes while maintaining birefringence at the 10 −6 level, which provides more degrees of freedom in the design of few-mode fiber employment in space division multiplexing (SDM) systems. Our simulation analysis demonstrates that the effective refractive index difference (Δ n eff ) between the spatially degenerate modes in this fiber falls within the range of (2.44−6.42)×10 −4 at 1530–1565 nm, given the refractive index difference level typical of conventional fiber core and cladding. In addition, the polarization separation level for each mode is on the order of 10 −6 and below. The design requirements of significant separation of spatial degeneracy and basically no separation of polarization degeneracy are achieved. Based on the characteristics of spatially degenerate mode in this fiber, we extend the regulation law for spatially degenerate mode separation of the LP m n mode groups in center-assisted ring-core fibers. In comparison to the existing center-assisted ring-core fiber structure, the ECRF can further reduce the fabrication difficulty and increase the feasibility of preparation.